CN116368158A - Efficient green method for preparing nanocellulose, novel modified nanocellulose and application of novel modified nanocellulose - Google Patents

Abstract

The present invention relates to an efficient process for the preparation of nanocellulose using a mixture of ammonium formate and at least one acid as reactants and solvents, as well as to novel modified nanocellulose and its use.

Description

Efficient green method for preparing nanocellulose, novel modified nanocellulose and application of novel modified nanocellulose

Technical Field

The present invention relates to an efficient process for the preparation of nanocellulose using a mixture of ammonium formate and at least one acid as reactants and solvents, as well as to novel modified nanocellulose and its use.

Background

Cellulose is a linear polymer of beta (1, 4) -linked D-glucose units, which is the most readily available polymer on earth and is used in a variety of applications due to its biocompatibility, non-toxicity and excellent mechanical properties. The separation of cellulose, in particular from plant fibres, generally involves chemical treatments consisting of alkaline extraction and bleaching.

In the last decade, the preparation and new applications of so-called nanocellulose have attracted tremendous interest. The term nanocellulose is often used for cellulosic materials having at least one dimension on the order of nanometers. Their unique combination of cellulose properties and nanomaterial characteristics opens up a new field of material science.

Today, there are three main types of nanocellulose materials: bacterial Nanocellulose (BNC), mechanically layered Cellulose Nanofibers (CNF) and hydrolytically extracted Cellulose Nanocrystals (CNC) (see review below: klemm et al, nanocelloses: A New Family of Nature-Based Materials, angew.Chem.Int.Ed.2011,50,p.5438to 5466;A.Dufresne,Nanocellulose:a new ageless bionanomaterial,Materials Today,Vol.16,No.6,2013 and Klemm et al, nanocellulose as a natural source for groundbreaking applications in Materials science: today's state, materials Today, vol 21, number 7, 2018).

Although the manufacturing costs of BNCs are quite high due to the low space-time yield, BNCs are generally obtained in such high purity for application in medical applications even without cumbersome purification procedures. The most commonly used bacteria are acetic acid bacteria of the genus Acetobacter (Gluconobacter). During biosynthesis, cellulose chains are produced and accumulate in fibrils, which generally have a cross-sectional size of 2 to 20nm and a degree of polymerization of glucose units of 4,000 to 10,000. Such fibrils generally exhibit small numbers of defects or amorphous domains (amorphous domains).

CNF is most commonly and on a larger scale manufactured from delignified and preferably bleached pulp. For example, mechanical delamination of the fibers is achieved by using high pressure homogenizers, microfluidizers, conventional refiners, high speed blenders and extruders, or techniques such as ball milling, steam explosion and ultrasound. These methods are very simple but require high energy input, damage the fibers, and produce CNFs with a wide distribution of fibril diameters and lengths. In general, CNF exhibits a diameter of 5 to 60nm and a length of 100nm to 10mm, and a degree of polymerization of 500 or more.

The separation of CNC from wood pulp and cotton by acid hydrolysis using sulfuric acid was first reported in the 40 s of the 20 th century. It is well known that acids degrade more accessible and/or disordered cellulose domains, leaving the highly crystalline domains intact. CNCs typically have dimensions of 100 to 250nm in length and 5 to 70nm in diameter, and a degree of polymerization of 500 to 15,000.

Newer methods of separating CNCs include oxidation and hydrolysis with acids such as hydrochloric acid, hydrobromic acid, citric acid, or phosphoric acid. The choice of acid directly affects the colloidal and thermal stability, size and surface charge of the CNC. For example, phosphoric acid and hydrochloric acid hydrolysates produce CNCs with low or no charge content, and CNCs are generally aggregated but have higher thermal stability. It is therefore important to optimize the reaction conditions of each isolation procedure to ensure the preparation of stable and predictable nanomaterials. The most common starting materials for CNCs are wood pulp and cotton, also including algae, bacteria and tunicates (tunicates), as well as waste materials such as coconut shells, rice hulls, and banana pseudostems.

However, while nanocellulose has great potential in various applications, its main drawback of commercial application is very high energy consumption, in particular CNF and CNC, and their poor long-term stability and storability are also found to be key issues.

Accordingly, various attempts have been made to overcome these problems.

These attempts include pretreatment such as mechanical cleavage, acid hydrolysis, enzymatic pretreatment and introduction of charged groups by carboxymethylation or 2, 6-tetramethylpiperidin-1-oxyl (TEMPO) mediated oxidation, helping decomposition by electrostatic repulsion (see Klemm et al Nanocellulose as a natural source for groundbreaking applications in Materials science: today's state, materials Today, vol 21, number 7,2018 and references cited therein, US 2014/0155301A1, US 2015/0171679 and CN 102180979B).

Similar methods are disclosed by Watanabe et al in Cytotechnology 13 (1993) 107-114, wherein cellulose is chemically modified by introducing cationic surface charges such as trimethylammonium hydroxypropyl-groups, diethylaminoethyl-groups, aminoethyl-and carboxymethyl-groups.

Other leading edge methods include pretreatment or preparation methods using ionic liquids or eutectic solvents as reaction media (summaries are given in h.tadess and r.luque, advances on biomass pretreatment using ionic liquids, energy environment.sci., 2011,4,3913).

Novel modified nanocellulose containing guanidine groups are disclosed in Li et al, recyclable deep eutectic solvent for the production of cationic nanocelluloses, carbohydrate Polymers, vol.199,1,2018, p.219-227, which are prepared by a two-step procedure involving cationization of dialdehyde cellulose with aminoguanidine hydrochloride and glycerol (a eutectic solvent as a reagent and reaction medium), followed by mechanical decomposition. The starting material dialdehyde cellulose was prepared by oxidizing cellulose (bleached kraft pulp) with sodium periodate.

The oxidation and modification procedures use expensive chemicals and impair the mechanical integrity of the cellulose, thus preventing commercial use.

From S.Filonenko, A.Voelkel and m. antonietti, valorization of monosaccharides towards fructopyrazines in a new sustainable and efficient eutectic medium, green chem, 2019,21,5256 it is known to use ammonium formate as a reagent and reaction medium for the conversion of carbohydrates into valuable fine chemicals.

Despite the advances described above, there remains a need to provide an efficient, green process for preparing nanocellulose materials that starts with readily available compounds without involving toxic or hazardous reagents.

It is another object of the present invention to provide nanocellulose with increased stability, i.e. reduced tendency for irreversible agglomeration when applied as a dispersion or colloid.

Disclosure of Invention

According to one aspect of the present invention there is now provided a method of preparing nanocellulose, the method comprising at least the steps of:

a) Providing a mixture comprising i) ammonium formate, ii) at least one acid, and iii) at least one cellulose-containing feedstock

b) Heating the mixture provided in step a) at a reaction temperature of 100 ℃ or higher.

In other aspects, the invention includes nanocellulose obtained by the above method and uses thereof.

Detailed Description

The invention also includes all combinations of the preferred embodiments, ranges parameters, and ranges or parameters disclosed in the following or the broadest disclosure.

Whenever the terms "including", "e.g." (e.g. ")", "such as" and "etc." are used herein, they refer to the meaning of "including but not limited to" or "e.g." but not limited to ", respectively.

As used herein, the term nanocellulose refers to polymer particles comprising β (1, 4) -linked D-glucose units, having an average degree of polymerization of at least 50D-glucose units and at least one dimension smaller than 1000 nm. Such nanocellulose may or may not be chemically derivatized.

In one embodiment, the average degree of polymerization is from 100 to 15,000, preferably from 200 to 10,000.

For the avoidance of doubt, the specification "at least one dimension less than 1000 nm" includes particles having an average cross-section in the range 3 to 200nm, preferably in the range 5 to 100nm, more preferably in the range 5 to 30nm and most preferably in the range 5 to 20nm and an average length in the range 15 to 5000nm, preferably in the range 50 to 1000nm, more preferably 70 to 800 nm.

In one embodiment, the aspect ratio, i.e. the ratio of the length to the cross section of the nanocellulose, is greater than 1, preferably 2 or more, more preferably 2 to 100 or 2 to 50.

In step a) of the process, a mixture is provided comprising i) ammonium formate, ii) at least one acid and iii) at least one cellulose-containing feedstock.

Suitable acids include organic acids such as organic compounds with one, two or three carboxylic acid (-COOH) or sulfonic acid groups, and inorganic acids such as sulfuric acid, hydrohalic acid, perhalic acid and phosphoric acid.

Preferred acids are monocarboxylic and dicarboxylic acids such as formic acid, acetic acid, propionic acid, glycolic acid, lactic acid, oxalic acid, levulinic acid, malonic acid, succinic acid, malic acid, maleic acid and adipic acid, with formic acid, propionic acid, glycolic acid, lactic acid, levulinic acid and succinic acid being even more preferred.

An important finding of the present invention is that the mixing of ammonium formate and organic acid results in a significant reduction in the melting point of the mixture compared to the individual components, so that the mixture can be used as both a reagent and a solvent without the addition of further other solvents. These so-called eutectic mixtures help to handle and increase the solubility of the cellulose-containing feedstock.

In one embodiment, for example, the molar ratio between ammonium formate and the sum of the acids is 0.2 to 1000, preferably 0.5 to 10.0, more preferably 1.0 to 5.0, and even more preferably 2.0 to 2.5.

Higher and lower molar ratios are in principle possible, but offer no advantage.

Thus, the invention also includes the use of ammonium formate and its mixtures with organic acids for the preparation of nanocellulose.

The mixture provided in step a) further comprises a cellulose-containing feedstock.

As used herein, a cellulose-containing feedstock includes any feedstock that contains cellulose, whether or not it is combined with lignin and/or hemicellulose and/or other structural building blocks.

Examples include microcrystalline cellulose, microbial cellulose, cellulose derived from marine or other invertebrates, recycled or waste paper (such as office waste paper and municipal waste paper), wood pulp (such as softwood pulp and hardwood pulp, whether bleached or not), chemical (dissolving) pulp, delignified pulp, pulp waste, natural biomass in the form of plant fibers, wood chips, sawdust, straw, leaves, stems or hulls, and cellulose synthetic fibers (such as tire cord) and other sources of cellulose (such as mercerized cellulose). Other examples include bagasse, miscanthus and bamboo.

The cellulose-containing feedstock may or may not be chemically derivatized by, for example, carboxymethylation, carboxylation, oxidation, sulfation, or esterification.

The cellulose-containing feedstock may or may not be mechanically pretreated by, for example, cutting, delamination, high pressure homogenization, sonication, or other known methods, or pretreated by enzymatic hydrolysis.

However, in one embodiment, the cellulose-containing feedstock is not chemically derivatized, enzymatically or mechanically pretreated.

Specific examples of cellulose-containing raw materials include bleached softwood pulp, microcrystalline cellulose (e.g., avicel PH-101), and pulp obtained from uncoated delignified paper.

In one embodiment, for example, the weight ratio between the cellulose-containing feedstock and the sum of ammonium formate and at least one acid is from 0.001 to 1, preferably from 0.01 to 0.25, more preferably from 0.02 to 0.20, even more preferably from 0.03 to 0.10.

Unless otherwise specifically indicated, the amounts of cellulose-containing raw materials are given and calculated on the basis of their dry weight, even though they generally contain different amounts of (residual) water.

The reaction was found not to be very sensitive to the presence of water. As a result, a certain amount of water in the reaction mixture provided in step a) is tolerable.

Thus, in one embodiment, the sum of ammonium formate, at least one acid, cellulose-containing raw material and water is 80-100wt. -%, preferably 90-100 wt. -%, in another embodiment 95-100 wt. -%, with respect to the total weight of the mixture provided in step a), the remainder typically being impurities from the starting materials used.

Providing a reaction mixture comprising the above compounds may be carried out in any manner known to the person skilled in the art, in any order of addition and in any vessel known to the person skilled in the art, to allow the reaction as defined above.

In step b), the reaction temperature is 100 ℃, preferably 140 ℃ or higher, more preferably 155 ℃ or higher.

In one embodiment, the reaction temperature is in the range of 100 ℃ to 190 ℃, preferably 140 ℃ to 185 ℃, more preferably 155 ℃ to 180 ℃, especially 160 ℃, 170 ℃ or 180 ℃.

Ammonium formate is known to start to decompose at temperatures above 180 ℃, so higher temperatures as described above may occur, but the formation of undesirable byproducts (such as formamide) may increase. At temperatures below 100 ℃, the reaction becomes too slow to effectively obtain the desired nanocellulose.

The pressure conditions are not particularly limited, and the pressure in step b) may be 500hPa to 50MPa, preferably 1000hPa to 1MPa. However, due to the potential decomposition of ammonium formate and the formation of low boiling components such as water, formic acid or other organic acids, the reaction is carried out at a pressure established after limiting the reaction mixture in reaction step a) and heating it to the desired temperature (i.e. under isovolumetric or near isovolumetric conditions).

The process according to the invention, in particular step b) thereof, may be carried out in any vessel or reactor suitable for the purpose and known to the person skilled in the art.

Preferably, the reaction is carried out in an autoclave or reactor which allows the process to be carried out under isovolumetric or near isovolumetric conditions.

For example, the reaction time is at least 30 minutes, preferably at least 90 minutes, more preferably at least 2 hours.

In one embodiment, the reaction time is from 60 minutes to 48 hours, preferably from 90 minutes to 12 hours, even more preferably from 2 to 4 hours.

Longer reaction times are possible, but in practice do not add any advantage over shorter reaction times, although shorter reaction times may reduce the yield of desired nanocellulose.

In step b), a reaction mixture comprising the desired nanocellulose is obtained. The water, formic acid and other acids and volatile byproducts present (such as formamide) can be removed by simple washing with water and/or alcohol or by distillation, fractional distillation or vacuum to isolate nanocellulose.

Nanocellulose can be redispersed in water by vortex mixing or sonication to form colloids, dispersions or suspensions, which are also included in the present invention.

Formic acid and other acids and excess ammonium formate can be recycled to step a) if desired.

The nanocellulose obtained by the method according to the invention exhibits a higher zeta potential (zeta potential) compared to mechanically prepared nanocellulose, and the nitrogen content indicates that at least the reduced ends of at least some of the cellulose chains within the nanocellulose are chemically modified and thus new and comprised in the invention.

Without wishing to be bound by theory, it is hypothesized that in step b) ammonium formate reacts with the reducing ends of at least some of the cellulose chains to form, by reductive amination, a cellulose polymer comprising repeating units of formula (I) (typically for polymers in which cellulose is a β (1, 4) linked D-glucose unit) and terminal units of formula (II):

since cellulose-containing raw materials generally contain more or less structural defects depending on their origin and oxygen is generally not excluded during the processing and/or the proceeding of the reaction steps a) and b), during the reaction step b) other amino groups can be introduced into the cellulose chains of the nanocellulose by reductive amination of the aldehyde groups already present in the cellulose chains or produced by partial oxidation, explaining the typical nitrogen content observed for nanocellulose according to the invention as defined below.

As a macroscopic effect resulting from the amination, the nanocellulose according to the invention exhibits an exceptionally high stability when dispersed in water or as a colloid. Such dispersions and colloids are stable without forming substantial amounts of gels even after two weeks of storage at room temperature.

Nanocellulose also exhibits high crystallinity.

The electrokinetic potential of the nanocellulose according to the invention is typically in the range of 2.0 to 50.0mV, preferably 5.0 to 40.0mV, more preferably 8.0 to 35.0mV, as measured according to the procedure described in the experimental section below.

The nanocellulose typically has a nitrogen content of 0.2 to 2.0wt. -%, preferably 0.3 to 1.8wt. -%, as measured by elemental analysis according to the procedure described in the experimental part below.

The crystallinity index of nanocellulose is typically in the range of 70% to 100%, preferably 75% to 100%, as measured by X-ray diffraction according to the procedure described in the experimental section below.

The degree of polymerization of nanocellulose strongly depends on the cellulose-containing feedstock, but is typically 100 to 15,000 glucose units, and in another embodiment 500 to 5,000.

Nanocellulose according to the invention and colloids, dispersions and suspensions comprising them can be used for various applications. This includes their use in foods and beverages, for example as additives such as low calorie additives, thickeners, stabilizers (e.g. foam stabilizers) and texture modifiers, and as microcapsules or coatings for odor and taste protection.

They are more useful in technical applications such as membranes for fuel cells and supercapacitors, as conductive membranes, loudspeaker diaphragms, in packaging materials or as packaging materials, in water absorption or purification (such as hydrogel beads for removal of aqueous dyes), water filtration membranes, nanocomposite heavy metal sensors, aerogels, flocculants and nanocomposite filters for groundwater modulation (groundwater mediation), as reinforcing additives for synthetic polymers such as thermoplastics and elastomers.

Other technical applications include paper/board coating and reinforcement applications, additives for paints, adhesives, latexes and cements, as stimulation fluids, drilling fluids, completion fluids and spacer fluids, where the novel nanocellulose is used as a stabilizer, thickener, shear thinning agent, proppant or reinforcing agent.

Other applications include their use in cosmetic or pharmaceutical compositions, as well as biomedical applications, such as for drug delivery, tissue engineering, bone repair materials, biosensors, bioadhesives, and microcapsules.

Thus, the invention also includes a food, beverage, film, packaging material, water absorbing or purifying material, heavy metal sensor, aerogel, flocculant, enhanced synthetic polymer, paper, cardboard, paint, adhesive, latex, cement, stimulation fluid, drilling fluid, completion fluid, spacer fluid, cosmetic or pharmaceutical composition, tissue and bone repair material, biosensor and bioadhesive comprising the nanocellulose or a colloid, suspension or dispersion thereof according to the invention.

The main advantage of the present invention is to provide a very efficient and green process for the preparation of nanocellulose and new nanocellulose allowing the formation of highly stable dispersions and colloids.

Hereinafter, the present invention is illustrated by examples, which are not intended to limit the scope of the present invention.

Experimental part:

general information:

materials:

ammonium formate (. Gtoreq.98%) was purchased from Alfa Aesar, glycolic acid (. Gtoreq.98%), alfa Aeser, propionic acid (99.5%) was purchased from Fluca, levulinic acid (98+%) was purchased from Acros Organics, succinic acid (99.5%) was purchased from Roth, and lactic acid (90 wt% aqueous solution) was purchased from Acros Organics.

If not noted, all chemicals used were obtained without further purification.

Characterization.

Elemental analysis

Elemental Analysis (EA) was performed with a vario MICRO cube CHNOS elemental analyzer (Elementar Analysensysteme GmbH, langenselbold). The elements have been detected C, H, N and O with a Thermal Conductivity Detector (TCD) and sulfur with an infrared detector (IR). Each sample was measured twice and the average calculated.

Electromotive potential

Electrokinetic potentials based on electrophoretic light scattering were measured using Zetasizer Nano ZS from Malvern Instruments (Malvern, united Kingdom). The wet sample after centrifugal washing was diluted with distilled water to obtain about 1% (nano-) cellulose suspension. The suspension was placed in disposable folded capillary cells (DTS 1070). The electrophoretic mobility of the (nano-) cellulose suspension was measured using Malvern software and converted to electrokinetic potential according to Smoluchowski equation. For electrokinetic potential measurements, three measurements reported the average of the samples with 95% confidence.

TEM imaging

Transmission Electron Microscope (TEM) images were recorded on a Zeiss Libra 912 microscope operating at 120 kV. Negative staining was performed with 1% uranyl acetate in distilled water to obtain higher image contrast.

Crystallinity index

The crystallinity index of nanocellulose was calculated from XRD data as the ratio between the maximum intensity of (002) lattice diffraction (at 22.8 °) and the intensity of the amorphous region in the same cell (at 18.6 °), see also Segal et al Textile Research Journal, october 1959, p.786to794.

II preparation of nanocellulose

Experimental procedure

Preparation of A Low melting mixture

To obtain a low melting point mixture for use as solvent and reactant, dry Ammonium Formate (AF) was mixed with organic acid in a 2:1 molar ratio. The mixture was ground in a mortar or thoroughly mixed in a glass beaker. As the mixture gradually liquefies under milling/mixing, visual formation of the desired low melting point mixture is observed. To promote its formation, the mixture is kept at 60 ℃ in a sealed glass bottle with stirring for at least two hours or until the crystals completely disappear.

The cellulose-containing raw material used in the B reaction.

SP: bleached softwood kraft pulp obtained from Mercer pulp was decomposed in deionized water under continuous stirring at room temperature overnight.

DP: 5g of uncoated premium delignified paper from Inapa Deutschland was cut into about 1cm with scissors ² And placed in 1L glass bottles. 1L of deionized water was added and the contents were mixed overnight. The pulp thus obtained was filtered on a glass funnel filter and washed on the filter with deionized water and ethanol in sequence. The washed pulp was dried at 60 ℃ for 24 hours. MC: the commercially available microcrystalline cellulose (Avicel PH-101, cellulose content 100%) was used

C reaction conditions

The cellulose-containing feedstock was added to the corresponding low melting point mixture of ammonium formate and acid in a glass beaker. Transferring the resulting reaction mixture to

In a beaker. The further reaction is carried out in an autoclave reactor under static conditions (i.e. without stirring) or with stirring, as follows:

static: will be charged with the reaction mixture

Beaker is used->

The lid was sealed and placed in a stainless steel Parr reactor (autoclave). The autoclave was maintained at 180℃for 4 hours. The reaction was stopped by cooling the autoclave in an ice bath and the resulting product mixture was transferred to a glass beaker.

Stirring: will be charged with the reaction mixture

Beaker is used->

And (5) sealing by a gasket. Placing the beaker intoIn a stainless steel high pressure bench reactor with an internal stirring system. The reactor was heated to 180 ℃ and held at that temperature for 4 hours (if not otherwise indicated in table 1). The reaction mixture was stirred at 200 rpm. The reaction was stopped by cooling the reactor with a water cooling system. After cooling the reactor to room temperature, the product mixture was transferred to a glass beaker.

The composition of the reaction mixture used to prepare the nanocellulose according to the invention, the cellulose-containing raw materials used, and the reaction conditions are summarized in table 1:

table 1. Composition of the reaction mixture, amount and type of cellulose-containing raw materials used, and reaction conditions.

* Lactic acid is used in the form of a 90wt% aqueous solution

And D, washing.The product mixture obtained according to part c) was diluted with a few ml of distilled water, mixed with a spatula and sonicated in a laboratory sonicator for 30 minutes. The colloid was then precipitated and the supernatant removed by centrifugation at 10,000RPM for 5 minutes on an Avanti J-E centrifuge (Beckman Coulter) equipped with a JA-25.50 fixed angle rotor. The precipitate was washed using the following sequence: redispersed in water, vortexed for 30 seconds, sonicated for 30 minutes, centrifuged at 10,000rpm (the last three runs were 20,000 rpm) for 5 minutes, and the wash decanted. This procedure was repeated four times with water and twice with ethanol until a clear wash solution was obtained. The ethanol was exchanged for water and the sample was centrifuged at 25,000rpm, the supernatant was decanted and the final product was freeze dried to give a white to beige powder.

The characteristics of the nanocellulose obtained in examples 1 to 26 are summarized in table 2.

Table 2: characterization of nanocellulose

Fig. 1 and 2 show TEM images of nanocellulose prepared from microcrystalline cellulose according to example 1 described above. Thereby obtaining whiskers having a diameter of 10 to 20nm and a length of at most 200 nm.

Fig. 3 and 4 show TEM images of nanocellulose prepared from softwood pulp according to example 6 above.

Fig. 5 and 6 show TEM images of nanocellulose prepared from delignified pulp according to example 11 above.

All nanocellulose obtained according to the invention showed high stability of its hydrocolloid for at least two weeks, clearly showing that stability is increased by the formation of amino groups at the reducing end of the cellulose chain, which leads to an increase of the nitrogen content in the nanocellulose according to the invention.

Claims (28)

1. A method for preparing nanocellulose, comprising the steps of:

a) Providing a mixture comprising i) ammonium formate, ii) at least one acid and iii) at least one cellulose-containing feedstock,

b) At 100℃or higher, preferably 140℃or higher, and more preferably 155 DEG C

Or higher, heating the mixture provided in step a).

2. The method of claim 1, wherein the nanocellulose represents a polymer particle comprising β (1, 4) -linked D-glucose units, the polymer particle having an average degree of polymerization of at least 50D-glucose units, at least one dimension of less than 1000nm, and being chemically derivatized or not chemically derivatized.

3. The method according to claim 1 or 2, wherein the at least one acid comprises an organic acid such as an organic compound bearing one, two or three carboxylic acid (-COOH) or sulfonic acid groups, and an inorganic acid such as sulfuric acid, hydrohalic acid, perhalic acid and phosphoric acid.

4. A process according to any one of claims 1 to 3, wherein at least one acid is selected from monocarboxylic and dicarboxylic acids, such as formic acid, acetic acid, propionic acid, glycolic acid, lactic acid, oxalic acid, levulinic acid, malonic acid, succinic acid, malic acid, maleic acid and adipic acid, wherein formic acid, propionic acid, glycolic acid, lactic acid, levulinic acid and succinic acid are even more preferred.

5. The process according to any one of claims 1 to 4, wherein the molar ratio between ammonium formate and the sum of acids is for example 0.2 to 1000, preferably 0.5 to 10.0, more preferably 1.0 to 5.0, even more preferably 2.0 to 2.5.

6. The method of any one of claims 1 to 5, wherein the cellulose-containing feedstock is selected from microcrystalline cellulose; microbial cellulose; cellulose derived from the ocean or other invertebrates; recycled or used paper, such as office waste paper and municipal waste paper; wood pulp, such as softwood pulp and hardwood pulp, whether bleached or not; chemical (dissolving) pulp; delignified pulp; pulp waste; natural biomass in the form of plant fibers; wood chips; saw dust; straw; leaves; stems or shells; and cellulose synthetic fibers such as tire cords; and other sources of cellulose such as mercerized cellulose, bagasse, miscanthus, and bamboo.

7. The method according to any one of claims 1 to 6, wherein the cellulose-containing feedstock is chemically derivatized, or not chemically derivatized, by, for example, carboxymethylation, carboxylation, oxidation, sulfation or esterification.

8. The method according to any one of claims 1 to7, wherein the cellulose-containing feedstock is mechanically pretreated or not mechanically pretreated by e.g. cutting, delamination, high pressure homogenization, sonication or other known methods, or pretreated or not pretreated by enzymatic hydrolysis.

9. The method according to any one of claims 1 to 8, wherein the cellulose-containing feedstock is selected from bleached softwood pulp; microcrystalline cellulose such as Avicel PH-101; and pulp obtained from uncoated delignified paper.

10. The method according to any one of claims 1 to 9, wherein the weight ratio between the cellulose-containing feedstock and the sum of ammonium formate and at least one acid calculated on its dry weight is e.g. 0.001 to 1, preferably 0.01 to 0.25, more preferably 0.02 to 0.20, and even more preferably 0.03 to 0.10.

11. The process according to any one of claims 1 to 10, wherein the sum of ammonium formate, the at least one acid, the cellulose-containing feedstock and water is 80 to 100wt. -%, preferably 90 to 100wt. -%, and in another embodiment 95 to 100wt. -%, relative to the total weight of the mixture provided in step a).

12. The process according to any one of claims 1 to 11, wherein in step b) the reaction temperature is between 100 ℃ and 190 ℃, preferably between 140 ℃ and 185 ℃, more preferably 155 °c

To 180 ℃, in particular 160 ℃, 170 ℃ or 180 ℃.

13. The process according to any one of claims 1 to 12, wherein the pressure in step b) is 500hPa to 50Mpa, preferably 1000hPa to 1Mpa.

14. The process according to any one of claims 1 to 13, wherein the reaction time in step b) is at least 30 minutes, preferably at least 90 minutes, more preferably at least 2 hours.

15. The process according to any one of claims 1 to 13, wherein the reaction time in step b) is from 60 minutes to 48 hours, preferably from 90 minutes to 12 hours, and even more preferably from 2 to 4 hours.

16. The process according to any one of claims 1 to 15, wherein the nanocellulose is separated from the reaction mixture obtained in step b) by washing with water and/or alcohol or by removing volatiles, e.g. by distillation, fractional distillation or vacuum.

17. The process according to any one of claims 1 to 15, wherein formic acid and other acids and excess ammonium formate, when present, are recycled to step a).

18. Nanocellulose obtainable by the method according to any one of claims 1 to 17.

19. Nanocellulose comprising at least some cellulose polymers comprising repeat units of formula (I) and terminal units of formula (II):

20. the nanocellulose of claim 19 further comprising amino groups obtained by reductive amination of aldehyde groups of said cellulose polymer that are already present or produced by partial oxidation.

21. The nanocellulose of any one of claims 18 to 20 having an electrokinetic potential of 2.0 to 50.0mV, preferably 5.0 to 40.0mV, and more preferably 8.0 to 35.0 mV.

22. Nanocellulose according to any of claims 18 to 21 having a nitrogen content of 0.2 and 2.0wt. -%, preferably 0.3 to 1.8wt. -%.

23. Nanocellulose according to any one of claims 18 to 22 having a crystallinity index measured by X-ray diffraction in the range of 70% to 100%, preferably 75% to 100%.

24. The nanocellulose of any one of claims 18 to 23 having a degree of polymerization of 100 to 15,000 glucose units or 500 to 5,000 glucose units.

25. Suspension, dispersion or colloid comprising nanocellulose according to any one of claims 18 to 24.

26. Use of nanocellulose as claimed in any of claims 18 to 24 or of the dispersion or colloid of claim 25 in food and beverage, for example as additives such as low calorie additives, thickeners, stabilizers such as foam stabilizers, and texture modifiers; and as microcapsules or coatings for odor and taste protection; as membranes for fuel cells and supercapacitors, as conductive membranes, loudspeaker diaphragms, in packaging materials or as packaging materials, in water absorption or purification such as hydrogel beads for removal of aqueous dyes, water filtration membranes, nanocomposite heavy metal sensors, aerogels, flocculants, nanocomposite filters for groundwater conditioning, as reinforcing additives for synthetic polymers such as thermoplastics and elastomers; for paper/paperboard coating and reinforcing applications, as additives for paints, adhesives, latex and cement; as stimulation fluids, drilling fluids, completion fluids, and spacer fluids; in cosmetic or pharmaceutical compositions and in biomedical applications such as for drug delivery, tissue engineering, bone repair materials, biosensors, bioadhesives and microcapsules.

27. A food, beverage, film, packaging material, water absorbing or purifying material, heavy metal sensor, aerogel, flocculant, reinforced synthetic polymer, paper, cardboard, paint, adhesive, latex, cement, stimulation fluid, drilling fluid, completion fluid, spacer fluid, cosmetic or pharmaceutical composition, tissue and bone repair material, biosensor and bioadhesive comprising nanocellulose according to any one of claims 18 to 24 or the dispersion or colloid according to claim 25.

28. Use of ammonium formate or of ammonium formate in combination with an organic acid for treating a cellulose-containing feedstock, in particular for preparing nanocellulose, such as nanocellulose according to any of claims 18-24.

Images